Frequency selective feedback amplifier



Sept. ,1951 R. 0. HUNTOON 2,566,333

FREQUENCY SELECTIVE FEEDBACK AMPLIFIER Filed June 24, 1946 2 Sheets-Sheet 1 &

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INVENTOR ROBE R T D. HUN TOON ATTORNEY R. D. HUNTOON FREQUENCY SELECTIVE FEEDBACK AMPLIFIER Sept. 4, 1951 2 Sheets-Sheet 2 Filed June 24, 1.946

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Int-I Rusk MQQIQ k6 s m lNV ENTOR ROBE T D. HUNTOON ATTORNEY Patented Sept. 4, 1951 l UNITED; STATES PATENT OFFICE FREQUENCY SELECTIVE FEEDBACK AMPLIFIER Robert D; Huntoon, Silver Spring, ;Md., assignor. to theUnited States of America as represented by theSecretary of the Navy ApplicationJune 24, 1946'; Serial No. 678,754

1 Claim.- (Cl. 179 171,)

(Granted -under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) Thefpresent invention relates to electrical sigantenna-ground system H, I2 for intercepting nal. translating systems and, while it has a conmodulated radio-frequency signals; a tuning syssiderable range of prospective application, it retem comprising a variable capacitor l3 and an inlates more particularly to audio-frequency amductor 14, coupled by a capacitor I 5 to a diode deplifiers of the type employing selective feedback 5 tector tube It; a detector. arrangement for deforobtaining desired frequency-response charriving the modulation components of received acteristics. radio-frequency signals and including the above- It isan objecthof the invention to provide an mentioned tube, which has an anode [l1 and 2. improved signal translating system of the charcathode I8; an amplifier including a pentode tube acter indicated and. having a relatively high in a gas-filled grid control tube of the Thyratron gain at a particular input-signal frequency or type 2|; and associated circuits which coopernarrow band of frequencies. ate with the above-described elements to cause It is also an object of. the invention to provide tube 2| to be triggered and to perform useful an improved signal translating system having work upon reception of signals having a desired a relatively sharp high frequency cutoff and if: audio-frequency characteristic intercepted by characterizedlby sharp discrimination against inthe antenna ground system H, l2, but which put. signals of frequencies higher than a desired cause tube 2| to remain nonconductive upon re critical frequency. ception of signals not so characterized.

It is av further. object of the invention to pro- Connected. between anode. l1 and ground is a vide an improved signal. amplifier incorporating 20 resistor 2 provided for the purpose of enabling few and conventional components. thedetector to lose charge and follow modulation.

It is an additional object of theinvention to The audio signals are brought out from the deprovide an improved signal amplifier having a tector to the amplifier through a resistor 22, very low gain for high. frequency input signals which cooperates with capacitor 21 to form a and a normal. gain for .low. frequency signals. 25 filter. that rejects radio-frequency signals 7 but A paramount object of the present invention is passes audio-frequency signals. Input signals to provide a signal amplifier which includes. a to be amplified are applied by impedance means feedbaclcnetworlr. having a transmission characto the input circuit of tube 20, which comprises teristicndependent upon frequency and is of light, a'cathodeElJ of the filamentary type and a concompact and .reliable construction. trol electrode 29. This impedance means includes For a better understanding of the present inresistor 22, a coupling capacitor 23, and a net vention, together. with other and further objects work comprising resistors 24, 25, the netthereof, reference is made to the. following dework being effectively disposed in shunt relascripticn taken in connection with the accomtionship with respect to the above-mentioned inpanying. drawingsin whichzqFig. 1 is a circuit put circuit. These three resistors have a comschematic of a complete electrical signal' transe mon junction. The terminal of resistor 26 relating system including a preferred embodiment mote from the junction is grounded and the reof thepresentvinvention; Fig. 2 isa circuit schemote. terminal. of resistor 25 is connected to an maticsimilar toFig. 1 and in which equivalent appropriate grid bias potential source (not circuit parameters are substituted for the diode shown) indicated at -C. An additional resistor detector and associated circuits there illustrated, 28, serially related to the network comprising recertain of the amplifier circuits being rearranged sisters 24, 25, and 26, is connected at one terfor. the. purpose of facilitating thedescription minal thereof to a control electrode 29 and at thereof; .Fig. 3 comprisesa set of curves showing M the other terminal thereof to the junction of the effects of various values of the feedback voltcondenser. 23 and resistor It will be underage fraction B on thefrequency-response .charstood that resistors 24, 25, and 26 maybe replaced acteristic of a typical amplifier of the type inbya single equivalent resistor. Resistors 25 and cluded in the Fig. 1 embodiment. 25 function as an expedient for dropping the po- Referring now specifically to Fig. .1, the signal tentiallof the; source,C to the desired grid bias; translating system there illustrated includes: an 0 Representative-of the types of vacuum tubes which may be employed as element 20 are the Sylvania SA781A and the Raytheon QF206. Tube 20 includes a suppressor grid electrode 33 connected to filament 30, a screen grid electrode 34. and an anode 3|. Screen 34 is coupled to an appropriate source of space current and anode potential (not shown) indicated at +3, through a screen dropping'resistor 35. In order to prevent the degenerative effect of screen voltage changes on amplifier gain an audio-frequency by-pass condenser 36 is provided between screen and ground. Anode potential is provided by coupling anode 3| through resistor 42 to source +B.

The output circuit of tube 20 is coupled through blocking and coupling capacitor 38, grid resistors 39 and 40, and resistor 4|, to a control electrode of the tube 2! thereby to apply the amplifier output thereto.

Filament heating currents for tubes [6, 20, and 2|, are furnished by appropriate current sources (not shown) indicated as +A, +A and +A". A choke 50 is included in circuit between source +A and filament 39.

The above-described elements are individually conventional in character and well known to those skilled in the art, so that further description thereof is deemed unnecessary. My invention resides in the feedback and signal mixing network now to be described and in its combiconnected to the control electrode of tube 20, is 3 provided. According to another feature of my invention the feedback signal means includes the portion 24, 25, and 26 of the impedance coupling means above described, the additional impedance 28 serially related to that portion, and resistancecapacitance elements for coupling the output circuit of tube 20 to its input circuit. Other features of my invention reside in the inclusion of the alternating current resistance of detector tube l6 as a parameter in a voltage divider network forming a part of the feedback signal means.

In accordance with the above-mentioned features, there is coupled to the output circuit of tube 20 a series capacitance-shunt resistance phase shifting and attenuating network having a transmission characteristic which varies with frequency. This network is similar in some respects to those described in the proceedings of the Institute of Radio Engineers, February 1941, pages 43 to 49, inclusive, an article by E. L. Ginzton and L. M. Hollingsworth, but differs therefrom in important respects presently to be pointed out. The last-mentioned network is coupled to plate resistor 42. It comprises three meshes. The first mesh consists of a capacitor 43 and a resistor 44. The second mesh consists of a capacitor 45 and a resistor 46. The third mesh consists of a capacitor 41 and a complex impedance. Referring now to Fig. 2 it will be seen that this impedance includes all of the elements between the terminal of resistor 48, remote from that connected to condenser 41, and ground. Specifically it includes, successively, resistor 48, resistor 23 in series relation thereto and two series-parallel branches, one of which includes the above-described elements 22, 23, 2|, and the alternating current plate resistance Rd of tube It, and the other of which includes the elements 24, 25, and 25. Thus this complex voltage-divider signal-combining network for coupling the output and input circuits of tube 20 includes as a parameter the plate impedance of the detector tube I6. In this manner the feedback signal is introduced across resistor 28 in series with resistor 24 and the parallel combination of resistors 25 and 2B and cooperates with the input signal applied across resistor 24 and this parallel combination to determine the frequency response characteristic of tube 20.

Referring now to the operation of the abovedescribed system it will be assumed that modulated radio-frequency energy is intercepted by the antenna ground system and applied to the detector tube l6, which in turn derives the modulation components thereof and applies an audiofrequency signal to the input of amplifying tube 20. The signal translating system has such operation that the gain of tube 20 is very low for audio frequencies above a critical frequency, but high at a predetermined frequency, so that tube 2| may be made to fire upon the reception of signals of the last-mentioned frequency and to remain nonconductive for received signals of frequencies in excess of the critical value.

An amplifier output signal appears across the plate resistor 42. It is applied to the Thyratron by way of condenser 38 and resistors. 39, 4D, and 4|. Output voltage appearing across resistor 42 is coupled back to the input circuit of the amplifier tube through the phase shifting network comprising elements 42, 43, 44, 45, 46, 41, 48, 28, 24, 25, 25, 23, 22, 2|, and the equivalent resistor RD, or alternating current plate impedance of the diode. There is one frequency for which the phase-shifting network introduces a phase shift of 180 degrees so that the feedback signal applied under this condition to the input circuit is in phase with the, input signal. At this frequency feedback increases the gain of the amplifier and acts regeneratively. At higher frequencies the phase shift is less than 180 and the attenuation of the feedback signal'is lesser, so that a strong out-of-phase signal is fed back to the input, thereby greatly to reduce the gain of the amplifier- This result follows from the fact i that thereactance of the series condensers de creases as frequency is increased. At lower frequency the transmission characteristic of the attenuating network is such that the phase shift is greater than 180 degrees so that the tendency of the feedback signal is then to reduce amplifier gain. However, at low frequencies the attenu ation of the feedback network is so great that the effect of feedback is negligible, and the low frequency response of the amplifier is similar to what it would be without feedback. By proper adjustment of the voltage division of the voltage divider network, sufficient feedback may be introduced to provide the desired shaping of the response characteristic without causing oscillation.

The impedance of the input circuit determines in part the amount of amplifier feedback and hence the gain thereof.

Referring now specifically to Fig. 3 of the drawing'there are illustrated curves showing the effect of various values of the fraction B of the output voltage, fed back through the feedback network to the amplifier input circuit, on the amplifier frequency-responsecharacteristic. These curves represent data empirically obtained from test of a circuit in accordance with Fig. 1 having the parameters hereinbelow listed in Table I. Table II contains altered values of. circuit components which may be employed for the purpose of making such. an amplifier peak at an input-signal audio frequency of 60 cycles per second. The

5 6 fraction B and the theory of feedback are defined Altered values for SA 781 A pentode and explained in detail in the following publica- Component; value tions: 1. Terman, Radio Engineering, New York. R3 megohm 4.7 1937, pages 243-256; 2. An article entitled Some :3 R30 do 1.0 Applications of Negative Feedback With Particu- Csa fll f 0.005

TABLE II Altered values of circuit components to make amplifier peak at 60 cycle sec.

QF 200 SA 781A Value Value c v 1 o 0 V 32% 5, Sit" (Effi 3223 5 35- 33 it? 0. 05 C23 0. 04 R35 5. 0 C35 0. 01 Gas-"u. .02 Ru.. 1.5 643.... .002 R:9. 1.0

3. 0 0.5 .002 R45 0. 75 0. 75 O4! 0005 3.0 o... .005 2.0

1 Replaced by l.5-volt battery to ground.

lar Reference to Laboratory Equipment, by F. E. Terman, R. R. Buss, W. R. Hewlett, and F. B. Cahill, Proceedings of the Institute of Radio Engineers, October 1939, pages 649-655; 3. A publication entitled Frequency Discrimination by Inverse Feedback, by George H. Fritzinger, Proceedings of the Institute of Radio Engineers, February 1938, pages 207-225; 4. Theory and Application of Electron Tubes, Reich, New York, 1944, pages 197-209 inclusive; 5. A publication en titled A New Type of Selective Circuit and Some Applications, H. H. Scott, Proceedings of the Institute of Radio Engineers, February 1938, pages 226-235.

While not proposing to limit the invention to any specific parameters, the following have been found practicable:

TABLE I Circuit pammeters-for QF 206 pentode Component: Value Rs megohm- 0.05 R22 0.10 R21 do 1 1.0 R24 do 1.0 R28 do 0.15 R43 d0 1.0 R35 d0 3.3 R42 d0 0.68 R44 d0 1.0 R46 d0 1.0 R do 1.0 R do 3.3 R39 "10--" 2.2 R40 d0 0.10 +A volts 1.2 C do --6.0 +13 do 135 -C do --6.0 Oz: .f 0.02 C36 d0 .01 C43 do .001 C45 do .001 Can do .002 C41 ,u;tf 250 Values and tolerances for circuit components Component: Value L 7.5-ohm choke While there has been shown and described what is at present considered to be a preferred embodiment of the invention, it will be obvious to those skilled in the art that various modifications and changes may be made therein without departing from the spirit thereof; and it is intended in the appended claim to cover all such changes and modifications as fall within the true spirit and scope or the invention.

The invention herein described may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

I claim:

A frequency selective amplifier having an input circuit of internal impedance variable with frequency, and an output circuit, said amplifier comprising an electron tube having an anode, cathode and control electrode, a first filter network, a series resistance unit connecting said control electrode to the input circuit through said filter network whereby to pass audio frequencies only, a second filter network interconnecting at least a portion of said output circuit with said input circuit at the junction of said resistance unit and control electrode, said second filter network including at least three stages of series capacitance, shunt resistance for a first and second capacitance of said stages, and a complex shunt impedance for said capacitance of said last stage including said series resistance unit and said internal impedance whereby to provide a frequency responsive voltage divider feedback circuit wherein the frequency variable impedance of said input circuit will in part determine the amount of feedback of said amplifier.

ROBERT D. HUNTOON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,173,426 Scott Sept. 19, 1939 2,178,072 Fritzinger Oct. 31, 1939 2,229,702 Larsen Jan. 28 1941 2,246,158 Worcester, Jr June 17, 1941 2,282,382 Root May 12, 1942 2,282,383 Root May 12:, 1942 

